confusion in Meta characters of perl

Question

When I run the below program, it gave me nothing ("") as result.
Scenario 1:-

$var = "A STITCH IN TIME SAVES NINE";
       if ($var =~ /[1..9]/i) {
         print "FOUND\n"
      }

But when I add space before and after '..' operator, it threw " Found and is " as a result.

Scenario :- 2

 $var = "A STITCH IN TIME SAVES NINE";
           if ($var =~ /[1 .. 9]/i) {
             print "FOUND\n"
          }

Could anyone explain me what is happening here??

Answer 1

The regex language is embedded in Perl (and vice versa), but it shares no syntax with Perl¹. This means other sytax for repetition or ranges.

_{1) Regexes share syntax with Perl strings, although the two are not fully compatible, see different meanings of the \b escape.}

Character classes define a set of multiple properties. The character class will match if one of the specified properties will match. A charclass can contain:

• single characters, like [aeiou] (match lowercase vowels)
• ranges, to match continuous ranges of code points: [A-Z] (uppercase latin characters)
• negation of the whole charclass: [^'] (everything that is not a single quote)
• named charclasses like \d, \w (and a lot of fun with Unicode properties)
• (POSIX charclasses)

If a charclass contains a character multiple times, this is irrelevant, as it behaves like a set union.

The metacharacters in charclasses are

• ]: end of match. To match square brackets, one has to do [\[\]] or [][], as a charclass cannot be empty.
• The negation operator ^ which is only special in leading position: [~&|^] would match any of the bitwise logical Perl operators.
• The range operator -. To match a literal minus, it can be put at the end of the charclass: The class [+-*] would be invalid (* comes before +, so the class is empty, which is illegal), but [+*-] works just fine
• The backslash still is the escape character.

Space is significant inside charclasses, even under the /x flag.

On your charclasses:

• [1..9] could also be written as [19.], and matches a 1, 9 or ., as the period is not a metacharacter inside charclasses.
• [1 .. 9] could be written [19. ], and additionally matches a space. As I said above, whitespace is significant in charclasses.

What you probably meant:

If you want to match any of the digits 0 or 1 to 9, you can use the range [0-9]. Remember that the minus is the range operator in charclasses.

Answer 2

When I run the below program, it gave me nothing ("") as result.

The string does not contain a 1, a full stop, or a 9. The pattern does not match anything in the string.

But when I add space before and after '..' operator, it threw " Found and is " as a result

It does contain a multiple spaces. When you expand your character class to match against to include spaces, it matches.

I have also include "$i" in the statement, but still it did not showed any value.

You never define $i. You only try to read from it.

Answer 3

The ".." does not indicate a range in your character class, it indicates a full stop (twice :-)). You don't have either a 1 , full-stop or 9 in your string, so it does not match first time round.

Also, you are not populating $i anywhere. $i is just a regular scalar variable, it does not get magically populated with anything. The i flag on the end of your regular-expression (/i) means case-insensitive search (in case you thought that was populating $i